The Exterior of the Fish 21 
essentially like cycloid scales. These are found in the garpike, 
and in many genera of extinct Ganoid and Crossopterygian 
fishes. In the line of descent the placoid scale preceded the 
ganoid, which in turn was followed by the 
cycloid and lastly by the ctenoid scale. Bony 
scales in other types of fishes may have noth- 
ing structurally in common with ganoid scales 
or plates, however great may be the superficial 
resemblance. 
The distribution of sceles on the body may 
vary exceedingly. In some fishes the scales 
Fie. 14.—Cyeloid are arranged in very regular series; in others 
ae they are variously scattered over the body. 
Some are scaly everywhere on head, body, and fins. Others 
may have only a few lines or patches. The scales may be 
everywhere alike, or they may in one part or another be greatly 
modified. Sometimes they are transformed into feelers or tactile 
organs. The number of scales is always one of the most valu- 
able of the characters by which to distinguish species. 
Lateral Line.—The lateral line in most fishes consists of a 
series of modified scales, each one provided with a mucous tube 
extending along the side of the body from the head to the caudal 
fin. The canal which pierces each scale is simple at its base, but 
its free edge is often branched or ramified. In most spiny-rayed 
fishes it runs parallel with the outline of the back. In most 
soft-rayed fishes it follows rather the outline of the belly. It is 
subject to many variations. In some large groups (Gobiide, 
Pecilude) its surface structures are entirely wanting. In scale- 
less fishes the mucous tube lies in the skin itself. In some 
groups the lateral line has a peculiar position, as in the flying- 
fishes, where it forms a raised ridge bounding the belly. In 
many cases the lateral line has branches of one sort or another. 
It is often double or triple, and in some cases the whole back 
and sides of the fish are covered with lateral lines and their 
ramifications. Sometimes peculiar sense-organs and occasionally 
eye-like luminous spots are developed in connection with the 
lateral line, enabling the fish to see in the black depths of the 
sea. These will be noticed in another chapter. 
The Lateral Line as a Mucous Channel.—The more primitive 
